Use of hearing devices such as external hearing aids, implanted hearing aids, cochlear implants, middle ear implants and electro-acoustic devices are widely used to deliver hearing capability to recipients. Typically, implantable hearing aids like cochlear implants are semi-implantable in that they comprise an implant with an electrode or a transducer and an external part. This external part typically comprises a hearing aid like device, which houses one or more microphones, a speech processor and the battery. The external device sends power and data to the implant via a transmitter coil. The transmitter coil and implant are positioned relative to each other by two magnets which reside in each component.
An important development in these types of hearing devices is the adaptation to a totally implantable form where these devices have no external components. This is seen to have a number of important advantages including being more aesthetically pleasing to the recipient as no components such as a microphone are required to be worn externally by the recipient. These fully implantable devices also have the advantages of being inherently waterproof and of providing hearing capability even while the recipient is sleeping, thereby providing an added safety benefit.
An important requirement of a fully implantable hearing device is the use of an implantable microphone or other sound detection device that functions to receive sound and process this into an electrical signal to drive the hearing prosthesis. A typical example of an implantable, subcutaneous, microphone involves the coupling of a transducer (e.g. electret, piezo or other pressure sensitive transducer) to an open ended cavity or acoustic volume located in a shell casing or housing formed from titanium, where the acoustic volume is covered by a diaphragm also typically formed from titanium. The diaphragm diameter and thickness and acoustic volume are then optimised to maximise the signal level after implantation. The subcutaneous microphone is also hermetically sealed so as not to present an infection risk to the recipient or otherwise expose the recipient to any toxic materials that the microphone is formed of.
Implantable microphones of this type have a number of associated disadvantages. As the subcutaneous microphone is covered by a layer of skin or tissue this directly attenuates the external sound resulting in a deterioration of the microphone's performance. More importantly, the skin layer decreases the resonance frequency of the diaphragm, since it will behave as a spring mass system. Due to this shift in resonance frequency the overall bandwidth of the microphone is reduced. Apart from the direct attenuation of external sound, there are a number of sources of noise in addition to the standard electrical noise associated with the electrical components which can also significantly degrade the performance of an implanted microphone. As the diaphragm is mass loaded, sitting under a layer of tissue or skin, it is sensitive to body induced vibrations transmitted typically via the skull which originate from the recipient's chewing, breathing, muscle movements, speaking, etc.
Another example of an implantable microphone involves direct measurement of components of the middle ear. In systems of this type, a sensor is implanted into the ear at a location to measure the mechanical vibration of one or more of the ossicles (i.e. the malleus, incus and stapes). Vibration sensors mounted to the ossicles are able to take advantage of extracting sound information from the natural auditory pathway, thereby potentially capturing a more natural sound spectrum at an improved signal to noise ratio. However, this approach has the significant disadvantage that more complex surgery is required for the recipient due to the additional fixation of the sensor to the middle ear.
It is desirable to improve upon any one or more of the above identified shortcomings.